1. Field of the Invention
The invention relates to a high-pressure fuel pump for a fuel injection system of an internal combustion engine, having at least one pump element, having a prefeed pump, having a metering valve, the prefeed pump pumping fuel from a tank to the suction side of the pump element or elements, and the fuel quantity pumped by the prefeed pump to the suction side of the pump element or elements being regulatable by the metering valve, and having a throttle for limiting the fuel outflow from the suction side of the pump element or elements, and to a fuel injection system embodying such a fuel pump.
2. Description of the Prior Art
For regulating the pumping quantity of a high-pressure fuel pump, a metering valve is disposed on the suction side of the pump elements of the high-pressure fuel pump and brings about a more or less pronounced throttling.
In the overrunning mode of the engine, or when a motor vehicle is moving downhill, fuel should not be injected into the combustion chambers of the engine. For this reason, in the overrunning mode the metering valve is closed. Since even in the closed state the metering valve has a certain leakage, even in the overrunning mode some quantity of fuel, although slight, reaches the pump elements and is aspirated by them. The fuel pumped by the pump elements, which is at high pressure, flows for instance into the common rail of the fuel injection system, and it can accordingly cause an undesirably high pressure to be built up in the overrunning mode, since the injectors are not injecting any fuel into the combustion chambers.
To prevent this, it is known to provide a so-called xe2x80x9czero-feedxe2x80x9d throttle, through which small quantities of fuel can flow from the suction side of the pump elements out into a fuel return or to the suction side of the prefeed pump. This prevents a pressure from building up on the suction side of the pump elements and thus prevents the pump elements from being able to overcome the closing force of the suction valves. Consequently in the overrunning mode the pump elements aspirate no fuel, and the unwanted buildup of pressure in the common rail during the overrunning mode is averted.
A disadvantage of this provision is that the zero-feed throttle is opened even upon starting of the engine, and thus the pressure buildup on the suction side of the pump elements is delayed or more difficult. Consequently the engine requires a high starting rpm and does not start until after a certain amount of time.
The object of the invention is to furnish a high-pressure fuel pump for a fuel injection system, as well as a fuel injection system, for an internal combustion engine, which when they are used improve the starting performance of the engine.
This object is attained according to the invention by a high-pressure fuel pump for a fuel injection system of an internal combustion engine, having at least one pump element, having a prefeed pump, having a metering valve, the prefeed pump pumping fuel from a tank to the suction side of the pump element or elements, and the fuel quantity pumped by the prefeed pump to the suction side of the pump element or elements being regulatable by the metering valve, and having a controllable throttle for limiting the fuel outflow from the suction side of the pump element or elements.
In the high-pressure fuel pump of the invention, the throttle can be closed during starting, so that the pressure buildup on the suction side of the pump elements is completed faster and the starting performance of the engine is thus improved.
In a variant of the invention, it is provided that the throttle closes when the fuel pressure on the compression side of the prefeed pump is below a first reference value, so that in all operating states in which the fuel supply to the high-pressure fuel pump is inadequate, the throttle is closed, and hence the entire amount of fuel pumped by the prefeed pump is available to the pump elements.
In a further feature of the invention, the throttle is controlled by a control valve that is subjected to the fuel pressure on the compression side of the prefeed pump, so that the control of the throttle is accomplished in a simple, reliable way. A control unit of the fuel injection system is not needed for this purpose.
In a further refinement of the invention, it is provided that the control valve also controls the inflow of fuel into the high-pressure fuel pump for the sake of lubrication, so that both functions, namely the control of the throttle and of the inflow of fuel into the high-pressure fuel pump for lubrication, can be accomplished simply and economically with one valve.
In a further feature of the invention, it is provided that the control valve is a cascade valve, and that the control valve controls the inflow of fuel into the high-pressure fuel pump for the sake of lubrication in a plurality of stages, thus on the one hand assuring that the high-pressure fuel pump is always adequately lubricated and on the other that there is enough fuel available to the pump elements at low rotary speeds.
It is especially advantageous if the throttle is integrated with the control valve, so that the number of component groups and connecting lines required is less, and less space is needed.
Further features of the invention provide that the throttle communicates on the outlet side with a return line that discharges into the tank, or with the suction side of the prefeed pump, or with the lubrication of the high-pressure fuel pump, so that regardless of how the throttle is connected on the outlet side, the advantages of the invention can be exploited.
The object stated above is also attained according to the invention by a fuel injection system for an internal combustion engine, having a high-pressure fuel pump, wherein the high-pressure fuel pump has at least one pump element, having a prefeed pump, having a metering valve, wherein the prefeed pump pumps fuel from a tank to the suction side of the pump element or elements, and the fuel quantity pumped by the prefeed pump to the suction side of the pump element or elements is regulatable by the metering valve, having a common rail communicating with the compression side of the high-pressure fuel pump, wherein the common rail can be made to communicate with a fuel return via a pressure regulating valve, and wherein in the overrunning mode of the engine, the pressure regulating valve is opened and the metering valve is closed.
In this fuel injection system, a zero-feed throttle can be dispensed with, since in the overrunning mode the pressure regulating valve is opened, and thus the pressure in the common rail drops so far that a pressure buildup in the overrunning mode is impossible. The fuel quantity pumped by the high-pressure fuel pump in the overrunning mode is equivalent to the leakage from the metering system and is very slight. An especially advantageous feature of the fuel injection system of the invention is that the function of the zero-feed throttle is achieved by means of a suitable triggering of the pressure valve that is present anyway and the metering valve that is also present anyway. This simplifies the structure of the fuel injection system and enhances its reliability.
In other features of the fuel injection system, it is provided that the pressure valve is a blocking or flow valve, and/or that a control unit is provided for controlling the fuel injection system, so that depending on the concept of regulation in the fuel injection system, the pressure in the common rail can be controlled by a blocking valve or a flow valve.
In another embodiment of the invention, the prefeed pump is a geared pump, and the prefeed pump is driven by the high-pressure fuel pump or by the engine. Since in high-pressure fuel pumps of this design, the rpm and thus the pumping capacity of the prefeed pump depend directly on the rpm of the engine, the pressure buildup on the suction side of the pump elements upon engine starting proceeds relatively slowly, so that the advantages of the high-pressure fuel pump of the invention and of the fuel injection system of the invention are especially advantageously attained.